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Flat Orientation of Hydrophobic Cores Induced by Two-Dimensional Confinement of Flexible Bolaamphiphiles at the Air-Water Interface Norihiro Mizoshita and Takahiro Seki* Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan Received August 5, 2005. In Final Form: September 21, 2005 We designed and synthesized a new bolaamphiphile consisting of a biphenyl core, flexible trisiloxane spacers, and terminal ammonium groups. The compression of the trisiloxane-containing bolaamphiphile at the air-water interface led to the formation of monolayer films with the hydrophobic rigid core lying flat on the film surface. Such monolayer structures were formed through the compression-induced conformational change of the flexible bolaamphiphile from an extended state to a folded one as confirmed by surface pressure-area isotherm measurements, water contact angle measurements, atomic force microscopy, and UV-vis spectroscopy.
Molecular integration using the Langmuir-Blodgett (LB) technique is a powerful tool for preparing organized molecular films with controlled molecular orientations.1-3 Ordered monolayer films obtained by the LB method have been intensively studied for the development of electronic and optical molecular devices, molecular recognition, sensors, and surface-modification agents.1,2 Proper molecular design of amphiphiles spread on water surfaces leads to the formation of a variety of monolayer structures (Figure 1).4-24 The monolayer formation of amphiphiles * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Petty, M. C. Langmuir-Blodgett Films; Cambridge University Press: Cambridge, U.K., 1996. (2) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (3) (a) Langmuir, I. J. Am. Chem. Soc. 1916, 38, 2221. (b) Langmuir, I. J. Am. Chem. Soc. 1917, 39, 1848. (c) Blodgett, K. J. Am. Chem. Soc. 1934, 56, 495. (d) Blodgett, K. J. Am. Chem. Soc. 1935, 57, 1007. (4) Shimomura, M.; Kunitake, T. Thin Solid Films 1985, 132, 243. (5) Wang, R.; Jiang, L.; Iyoda, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. Langmuir 1996, 12, 2052. (6) Schro¨ter, J. A.; Plehnert, R.; Tschierske, C. Langmuir 1997, 13, 796. (7) Badis, M.; Guermouche, M. H.; Bayle, J.-P.; Rogalski, M.; Rogalska, E. Langmuir 2004, 20, 7991. (8) Matsuzawa, Y.; Seki, T.; Ichimura, K. Thin Solid Films 1998, 327-329, 87. (9) Amabilino, D. B.; Asakawa, M.; Ashton, P. R.; Ballardini, R.; Balzani, V.; Belohradsky, M.; Credi, A.; Higuchi, M.; Raymo, F. M.; Shimizu, T.; Stoddart, J. F.; Venturi, M.; Yase, K. New J. Chem. 1998, 959. (10) Youm, S.-G.; Paeng, K.; Choi, Y.-W.; Park, S.; Sohn, D.; Seo, Y.-S.; Satija, S. K.; Kim, B. G.; Kim, S.; Park, S. Y. Langmuir 2005, 21, 5647. (11) Yabe, A.; Kawabata, Y.; Niino, H.; Matsumoto, M.; Ouchi, A.; Takahashi, H.; Tamura, S.; Tagaki, W.; Nakamura, H.; Fukuda, K. Thin Solid Films 1988, 160, 33. (12) Ariga, K.; Nakanishi, T.; Terasaka, Y.; Tsuji, H.; Sakai, D.; Kikuchi, J.-i. Langmuir 2005, 21, 976. (13) Yokoi, H.; Hayashi, S.; Kinoshita, T. Prog. Polym. Sci. 2003, 28, 341. (14) Bu¨gler, J.; Sommerdijk, N. A. J. M.; Visser, A. J. W. G.; van Hoek, A.; Nolte, R. J. M.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Am. Chem. Soc. 1999, 121, 28. (15) Lehmann, P.; Kurth, D. G.; Brezesinski, G.; Symietz, C. Chem.s Eur. J. 2001, 7, 1646. (16) Hayashi, H.; Abe, J.; Yoshida, H.; Asaoka, S.; Iyoda, T. Trans. Mater. Res. Soc. Jpn. 2003, 28, 569. (17) Fuhrhop, J.-H.; Fritsch, D. Acc. Chem. Res. 1986, 19, 130. (18) Fuhrhop, J.-H.; Wang, T. Chem. Rev. 2004, 104, 2901. (19) Kellner, B. M. J.; Cadenhead, D. A. J. Colloid Interface Sci. 1978, 63, 452. (20) Menger, F. M.; Wrenn, S. J. Phys. Chem. 1974, 78, 1387.
Figure 1. Schematic illustration of monolayer structures: (a) monolayers of conventional amphiphiles with rodlike and disklike cores; (b) monolayer behavior of normal bolaamphiphiles; and (c) monolayer behavior expected for a bolaamphiphile used in the present study.
consisting of functional rodlike or disklike cores and aliphatic chains has been reported to give ordered 2D molecular assemblies with different orientations of the cores such as the side-by-side stacking of upright rods and an edge-on or face-on orientation of disks (Figure 1a).4-10 In those monolayer films, alkyl chains cover the uppermost surfaces, and the functional cores are buried within the films or located in the aqueous phases. (21) Lee, J.; Joo, H.; Youm, S. G.; Song, S.-H.; Jung, S.; Sohn, D. Langmuir 2003, 19, 4652. (22) Patwardhan, A. P.; Thompson, D. H. Langmuir 2000, 16, 10340. (23) Song, J.; Cheng, Q.; Kopta, S.; Stevens, R. C. J. Am. Chem. Soc. 2001, 123, 3205. (24) Janietz, D.; Katholy, S.; Brehmer, L. Thin Solid Films 1998, 327-329, 74.
10.1021/la052150z CCC: $30.25 © 2005 American Chemical Society Published on Web 10/08/2005
Letters
Our aim in the present study is to develop surfacetunable LB films with hydrophobic rigid cores lying flat on the film surface. If molecular films covered with flatlying core arrays are obtained, then we can expect novel surface-mediated functionalities of LB films as electroactive surfaces, alignment layers for liquid crystals, reaction fields for materials synthesis, and so forth.25-29 Simple material design to form such monolayer structures is to use bolaamphiphiles with their two terminal groups on the water surface.17-24 Bolaamphiphiles connecting the terminal hydrophilic groups with long alkylene spacers have been shown to form a reverse U-shaped conformation at the air-water interface.20-22 Two-dimensional assembly of such folded bolaamphiphiles leads to the integration of hydrophobic central parts of the bolaamphiphiles to the uppermost surface, which opens a new route for the design of monolayer materials. However, bolaamphiphiles with unsaturated spacers or π-conjugated central cores adopt extended molecular conformations on water surfaces, and the compression of the monolayers eventually leads to the formation of side-by-side molecular assemblies (Figure 1b) or the collapse of monolayer structures.17-19,22,24 This behavior is due to the rigidity of the spacer parts connecting the terminal hydrophilic groups and van der Waals interaction in the lateral directions of the molecules. The situations drawn in Figure 1a and b will impede the utility of the functional molecules because the photo- and electroactive cores are buried within the hydrocarbon chains. Here we propose a new molecular amphiphile design to obtain LB films covered with flat-lying hydrophobic cores. We designed bolaamphiphile 1 consisting of a biphenyl core, trisiloxane spacers, and terminal ammonium groups. Our intention for this compound is to form floating monolayers composed of flat-lying molecules with an extended conformation at low surface pressures and to lift up the core part by surface compression inducing a conformational change in 1 from an extended state to a folded one (Figure 1c). We adopted a quaternary ammonium salt structure for the hydrophilic groups because terminal parts are required for strong anchoring on the water surface. The key design of compound 1 is the introduction of trisiloxane spacers into the arm parts. In addition to their flexible and hydrophobic properties, siloxane moieties are known to exhibit surface activity.30,31 At low surface pressures, the surface-active properties of the hydrophobic spacers can aid the flat orientation of 1 with an extended conformation. On compression, bolaamphiphile 1 is expected to change its molecular conformation smoothly because of the flexibility of the spacers. Moreover, the siloxane moieties should suppress the crystallization of the compound, resulting in the restriction of the solidification of compressed molecular films.
Compound 1 was synthesized by the Pt-catalyzed hydrosilylation of 4,4′-diallyloxybiphenyl and 1-(4-bro(25) Boden, N.; Bushby, R. J.; Martin, P. S.; Evans, S. D.; Owens, R. W.; Smith, D. A. Langmuir 1999, 15, 3790. (26) Ichimura, K. Chem. Rev. 2000, 100, 1847. (27) Seki, T. Polym. J. 2004, 36, 435. (28) Aizenberg, J.; Black, A. J.; Whitesides, G. M. Nature 1999, 398, 495. (29) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature 1988, 334, 692. (30) Belousov, S. I.; Sautter, E.; Godovsky, Y. K.; Makarova, N. I.; Pechhold, W. Polym. Sci., Ser. A 1996, 38, 1008. (31) Mark, J. E. Acc. Chem. Res. 2004, 37, 946.
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Figure 2. (a) Surface pressure-area isotherm of 1 on water at 20 °C. Compression rate 30 cm2/min. The inset shows the plot of absorption intensity at 267 nm vs the reciprocal of the area per 1 on the water surface. (b) Contact-mode AFM image of the LB film of 1 transferred at a surface pressure of 5 mN/m onto a mica substrate. The central area (500 nm × 500 nm) was repetitively scratched by the AFM tip to remove the film.
mobutyl)-1,1,3,3,5,5-hexamethyltrisiloxane, followed by quarternization with triethylamine. The compound is a highly viscous liquid. The introduction of trisiloxane moieties should suppress the crystallization of the compound. We found that bolaamphiphile 1 formed a monolayer film on the water surface. Figure 2a shows the surface pressure-area isotherm of 1 at 20 °C. The surface pressure starts to lift at a molecular area of 3.5 nm2, which corresponds to the area of 1 lying with an extended conformation. On compression, a plateau region is observed at a surface pressure of 5-7 mN/m. This behavior implies a conformational change of 1 with a deformation of the trisiloxane moieties because the surface pressure is close to that of a plateau region of poly(dimethylsiloxane) (9 mN/m).30 On further compression, a steep increase in the surface pressure is observed when the molecular area is less than 1.0 nm2. The large limiting area (1.1 nm2) indicates that compound 1 is assembled with both hydrophilic ends on the water surface, that is, with a folded conformation as shown in Figure 1c. UV-vis spectroscopy measurement on the water surface showed that the biphenyl cores of 1 exhibited neither sideby-side association nor orientational change on compression. For compound 1 spread on the water surface, the absorption band of the biphenyl core is observed at 267 nm at zero surface pressure. The maximum absorption wavelength and the spectral shape are the same as those in the dilute chloroform solution of 1. During the compression process, both the absorption wavelength and spectral shape show no change. The biphenyl cores exist as monomeric species in the Langmuir films before and after compression. Moreover, the absorption intensity is in proportion to the reciprocal of the molecular area of 1 (Figure 2a, inset). These results indicate that the biphenyl cores lying in the monolayer composed of extended 1 maintain their flat orientation during the compression of the monolayer film. The Langmuir monolayers could be transferred onto hydrophilic mica substrates and alkali-washed quartz slides by the Langmuir-Blodgett vertical dipping method. The transfer ratios were close to unity when the surface pressures were not less than 5 mN/m (Table 1). For the well-transferred LB films, we first measured water contact angles to examine the surface properties. The water contact angles of the LB films transferred onto mica substrates at surface pressures of 5 and 10 mN/m are 70.3 and 74.8°, respectively, indicating the formation of
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Table 1. Surface and Spectroscopic Properties of the LB Films of 1 Transferred at Surface Pressures of 1-10 mN/m surface pressure (mN/m) 1 5 10
transfer ratio (%)
water contact angle (deg)a
>60 ∼100 ∼100
15.0 70.3 74.8
dichroic ratio at 267 nmb 0° 45° incidence incidence 1.01 1.01 1.01
1.47 1.46 1.47
a Measured for the LB films prepared on the mica substrates. Ratio of absorbance in the parallel and perpendicular directions to the dipping direction of the quartz substrates.
b
Figure 3. UV-vis spectra of the LB film of 1 transferred onto a quartz slide at a surface pressure of 10 mN/m.
Figure 4. Schematic illustration of the monolayer structures of 1.
hydrophobic surfaces (Table 1). We also measured the film thickness by atomic force microscopy (AFM). Figure 2b shows an AFM image of the LB film transferred at 5 mN/m after repetitively scratching the central area by the AFM probe to remove a part of the film. The thickness of the LB film is estimated to be ca. 1.2 nm, which is much shorter than the molecular length of 1 with an extended conformation (ca. 4.5 nm). The LB film prepared at 10 mN/m showed a similar thickness. These results on the contact angle and the film thickness support the fact that the compression of the monolayers of bolaamphiphile 1 induces its conformational change from an extended state to a folded one, maintaining the hydrophobic core parts located on the film surface and the terminal hydrophilic groups on the substrate surface (Figure 1c). The small film thickness also suggests that the arm parts are not completely extended but partially bent. The flexible bent arms fill the space formed beneath the cores, resulting in the stabilization of the monolayer structures. In-plane and out-of-plane orientations of the biphenyl cores in the LB films were examined by polarized UV-vis spectroscopy. Figure 3 shows the UV-vis spectra of the LB film transferred at a surface pressure of 10 mN/m. When the incident light is in the normal direction of the film (incident angle 0°), no anisotropy is observed between the directions parallel and perpendicular to the dipping direction. The in-plane orientation of the biphenyl cores is randomly distributed. However, when the polarized spectra are measured at the incident angle of 45°, we can observe dichroism for the s- and p-polarized light. The ratio of the s-polarized to p-polarized absorbance is 1.47 (Figure 3, Table 1). It is known that the dichroic ratio at an incident angle of 45° is expected to be 1.52 for a statistical planar distribution of all transition moments.32 The good agreement of the measured dichroic ratio with the theoretical value confirms the flat orientation of the
biphenyl cores in the LB films. We also performed the same measurements for the LB film transferred at a surface pressure of 1 mN/m, in which compound 1 is thought to be extended on the substrate surface. The dichroism of the film at incident angles of 0 and 45° is quite similar to that of the compressed films (Table 1), indicating the maintenance of the flat orientation of the cores before and after compression. These results are consistent with the core distribution indicated by UVvis spectroscopy on the water surface. The monolayer structures of bolaamphiphile 1 are schematically shown in Figure 4. The bolaamphiphiles can spread on the water surface with an extended conformation at low surface pressures. On compression, 1 changes its molecular conformation from an extended state to a folded one, resulting in the formation of monolayer films with the hydrophobic cores lying flat on the uppermost surface. Here the cores exist as monomeric species and show no in-plane anisotropy. In conclusion, the formation of organized molecular films with the hydrophobic cores lying flat on the film surface has been achieved by the proper design of the amphiphilic molecule and the monolayer film preparation using the LB technique. The molecular assemblies composed of folded bolaamphiphiles, which are not formed in the bulk state,33 can be obtained by the 2D confinement of 1 at the air-water interface. It should be noted that the core parts homogeneously cover the film surface and the density of the cores is tunable by the extent of compression. These surface-tunable monolayer films may be applicable to novel thin film materials including surface treatment of electrodes, alignment layers for ordered materials synthesis,
(32) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92.
(33) We have already found that 1 exhibits a lyotropic lamellar phase in the bulk aqueous solution. In this liquid-crystal phase, 1 adopts an extended conformation. X-ray diffraction measurement for the aqueous solution of 1 containing ca. 30 wt % of water showed that the layer spacing of the lamellar phase was 4.86 nm, which corresponds to the thickness of molecular layers consisting of 1 with an extended conformation and water.
Letters
and molecular sensors to recognize specific species.25-29,34,35 The introduction of various functional cores with two or more arms and the induction of in-plane order for monolayer films are now in progress. Acknowledgment. We thank Dr. S. Nagano, Dr. T. Hikage, Mr. Y. Ohnuma, and Mr. M. Hara for their (34) (a) Zhang, S.; Echegoyen, L. J. Am. Chem. Soc. 2005, 127, 2006. (b) Zhang, S.; Palkar, A.; Fragoso, A.; Prados, P.; de Mendoza, J.; Echegoyen, L. Chem. Mater. 2005, 17, 2063. (35) Liu, Y.; Mu, L.; Liu, B.; Kong, J. Chem.sEur. J. 2005, 11, 2622.
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experimental support. N.M. is thankful for financial support from the Research Fellowships of the Japan Society for the Promotion of Science (JSPS) for Young Scientists. Supporting Information Available: Experimental section, AFM images of the LB films, UV-vis spectra of 1, and results of some supplementary experiments. This material is available free of charge via the Internet at http://pubs.acs.org. LA052150Z
